There is growing interest in the 2f1-f2 acoustic distortion product (ADP) for both clinical and research applications. It is highly frequency specific, it can be measured in majority of human and animal subjects, and it can be used objectively for the hearing screening of infants. However, the success of its application is limited by the fact that the generation mechanism for the ADP is still not fully understood. There are questions about the site of generation of the ADP, whether it is generated within a single cochlear region, or whether multiple regions contribute to the ADP measured in the ear canal. Detail about how the resultant ADP is generated within a specific region is also poorly understood. It is not clear whether the overall ADP results from essentially a single generator or from a group of generators continuously distributed within an area wherein contributions are vectorially summed. The study proposed here will investigate these issues with a new approach in which the fine frequency structure of the ADP and its basilar- membrane-related characteristics will be examined. With the aid of a vector sum model of ADP fine structure (Sun et al., I994a,b), four experiments will be conducted. The first two experiments are designed to examine the effects of the frequency and level ratios of the primary tones on the ADP fine structure. These first experiments will address basic issues concerning the site and number of ADP generators. For example, does re-emission from the 2f1-f2 place contribute to the resultant ADP fine structure? The latter two experiments will yield data concerning the effects of noise maskers on the ADP fine structure. Emphasis will be placed on the way the ADP fine structure is generated. Changes in the ADP fine structure with varying level and bandwidth of a narrowband masker will help determine if there is a vector sum mechanism involved in the generation of ADP fine structure, and if so, its critical bandwidth. These experiments will test the published model and will extend our basic knowledge of otacoustic emissions.